JRuby is quite mature now, has a hell of a better performance than the official Ruby runtime (if you target Ruby 1.8.x; the 1.9.x branch impoved a lot, and makes performance differences between the 2 less relevant), and gives you very easy access to Java libraries.
Plus, if you way that you want to package WAR files then I guess it wouldn't be an issue for you to use a Java-enabled container/server.
So, I would definitely recommend that you get a closer look at JRuby.
And no, JRuby doesn't "have a JVM". It runs on top of a JVM. So you can develop your whole project in JRuby if you want (or even code it with Ruby and then call your code using JRuby, though you might encounter a few surprises so I'd recommend to use JRuby from the beginning) and just use the Java classes whenever you need to.
It's fairly well integrated, it has a lively and friendly community, and they produce regular updates to the implementation.
Regarding you build environment, if you use JRuby you will be able to call the compiler from Ant easily (like anything else, really), so that wouldn't be an issue. There might even be some custom tasks already implemented, I don't really know the state of things in this area.
Personal experience anecdotes and stuff: I used JRuby a lot for implementing automated test harness a few years back (mostly around 2007) and it wasn't even that mature at the time, and yet it was already in my opinion a much better alternative over the official Ruby implementation. Plus it was great in my case that it run on the JVM as I could also use it to unit test Java classes. And it's been improving steadily over time.
Its only disadvantage is that, if you use it to develop small programs or standalone tools, that you need to package your solution with a JVM or have the user install one, and that for short-lived programs the cost of firing up a JVM (especially older versions, it's quite fine now) might be a drag.
Apart from that, that was a very good solution and I still write scripts on it occasionally.